Cloud-surface interaction on the North Slope of Alaska

Arctic clouds play a substantial role in modulating global climate change. While considerable effort has been devoted to observing clouds and boundary layer structure over the Arctic Ocean and pack ice, much less attention has been paid to cloud-surface interactions over the tundra. The U.S. Department of Energy operates a research site on the North Slope of Alaska that is devoted to observing clouds and radiation transfer in the atmosphere. There are two major concentrations of instruments, one at Barrow on the coast and the other 100 km inland at Atqasuk, that provide an opportunity to analyze the characteristics and spatial variability of boundary layer clouds over the arctic land surface. We have analyzed three years of meteorology, radiation, and radiatively important cloud properties (such as liquid water path and cloud optical thickness) to see what role surface interactions may play in establishing cloud radiative characteristics. We have focused on summer (warm) clouds, since liquid water clouds provide the strongest radiative forcing. Just prior to the summer melt, surface sensible heat fluxes have a diurnal maximum of about 25 W/m2 at both sites. After the melt, these fluxes often approach 150 W/m2 over land but are nearly zero over the ocean. The land value appears to be adequate to couple boundary layer clouds to the surface. Because the tundra is covered in large part by lakes and melt ponds, the surface fluxes also provide a strong source of moisture that enhances the cloudiness. In contrast, the cold offshore water and ice appear to provide far weaker moisture fluxes. This difference is reflected in a dependence of the liquid water paths of low clouds on wind direction. Whether Barrow or Atqasuk has greater liquid water paths in boundary layer clouds depends on which site experiences the greater fetch over the wet tundra.